There are a wide variety of metal oxides and such metal oxides are used for various applications. Indium oxide is a well-known material and has been used for transparent electrodes required in liquid crystal display devices or the like.
Some metal oxides have semiconductor characteristics. The examples of such metal oxides having semiconductor characteristics are, for example, tungsten oxide, tin oxide, indium oxide, and zinc oxide. Thin film transistors in which channel formation regions are formed using such metal oxides are already known (for example, see Patent Documents 1 to 4, Non-Patent Document 1, and the like).
As metal oxides, not only single-component oxides but also multi-component oxides are known. For example, InGaO3(ZnO)m (m: natural number) having a homologous phase is known as a multi-component oxide semiconductor containing In, Ga, and Zn (for example, see Non-Patent Documents 2 to 4 and the like).
Furthermore, it is confirmed that an oxide semiconductor containing such an In—Ga—Zn-based oxide is also applicable to a channel formation region of a thin film transistor (for example, see Patent Document 5, Non-Patent Documents 5 and 6, and the like).